Even though as an artist you can make some pretty amazing designs for your word art by hand you may want to consider a design program such as CorelDraw, Illustrator, or even Manga Studios for your word art. You're going to save your self some time, especially if you decide to make changes. I hope by know you're starting to appreciate the job of a letterer. 
Word art for sound effects is pretty upfront. Size equates to loudness, so if your sound effect is really small its considered a whisper while if its really large its practically an explosion. Play with colors to contrast and enhance the scene. Font plays an important role in representing the sound displayed. There are several helpful sights when it comes to fonts and word art for comics. Try Blambot.com or Comic Craft for a great collection of fonts and advice about lettering.